Count the number of objects that match the specified \code{checks}.  This will
return the number of objects counted by the search.  If an error occurs during
the count, the count may reflect a partial count.  The real count will be higher
than the returned value.

A count is the server-side equivalent of performing a search, and counting the
number of objects that exist.  For that reason, its efficiency closely follows
that of search, except it will not transfer any objects over the network.
